Negative Lens: Spreads Light
Out.
Myopia corrected
Myopia is corrected with a negative lens that compensates for the
excess power of the cornea and lens. The focus point is moved
backwards.
Surgical Corrections
•
•
•
•
RK: small cuts around edge of cornea
PRK: laser reshapes surface of cornea
LASIK: laser reshapes internal layers of cornea
Implantable lens and rings
Hyperopia (Farsighted)
When the eye is hyperopic, the eye is too short and the
point of focus is in behind of the retina. A blurry image falls
on the retina.
Positive Lens: Focuses Light;
Magnifier
Hyperopia corrected
Hyperopia: Corrected
Hyperopia is corrected by placing an additional, positive lens in
front of the eye. The stronger the lens, the more the focus point is
moved forward.
Spherical Aberration
• Human cornea is “aspherical”
• Human lens has progressive index of refraction
Current Research Question:
• Why are some eyes myopic or hyperopic?
• Why are so many eyes emmetropic?
• Not random as after birth % emmetropic
increases.
• Most cases of myopia start later, in teens.
Associate with “close-work”.
• Two theories:
– Extra muscular forces for accommodation etc.
– Misguided growth
Astigmatism
Christman, 1971, p.150.
Radial
Zakia, 1997, p.220.
Accommodation
The Lens & Accommodation
The lens changes shape to focus objects at different distances.
Can also compensate for some myopia, hyperopia
Marmor & Ravin, 1997, p.16
Presbyopia
Presbyopia
Marmor & Ravin, 1997, p.17
Accommodation vs Age
• Fixed Focus
by 50
• “Presbyopia”
Gregory, 1997, p.39.
Hyperopia and Accommodation
Hyperopia & Accommodation
When young, hyperopia may compensated by
making the lens more round.
Myopia and Accommodation
Myopia & Accommodation
When young, myopia may not be obvious as it
can be compensated by flattening the lens.
The Lens and Aging
• Presbyopia
• Yellowing
• Opaque (Cataracts)
Cataract: young
.